Waste water streams from industrial plants, including petroleum refineries, chemical plants, pulp and paper plants, mining operations, electroplating operations, and food processing plants, can contain offensive substances such as cyanides, sulfides, sulfites, thiosulfates, mercaptans, disulfides, ammonia and mixtures thereof, that tend to increase the chemical oxygen demand (COD) of the waste water streams. Examples of these waste water streams in petroleum refineries include sourwater, sourwater stripper bottoms, and spent caustics and the term "waste water" is used herein to refer to the above described streams.
The Environmental Protection Agency (EPA) and various local agencies have placed limits on the allowable levels of these offensive substances in industrial waste water effluent streams. It is desired to develop a process suitable for treating these streams to meet the regulatory limits.
Methods for COD reduction by reducing the concentration in the waste water of one or more of these compounds include treatment of the waste water by precipitation, incineration, biological oxidation, and chemical oxidation using peroxoacids, H.sub.2 O.sub.2, Cl.sub.2, NaOCl, ClO.sub.2, and KMnO.sub.4, among others. Catalysts containing metals, such as copper, manganese, nickel, cobalt, tungsten, and chromium, among others, either in a soluble form or in a supported or complexed form, have also been mentioned as useful for waste water oxidation. It is also known that the addition of SO.sub.2 type compounds, such as SO.sub.2, alkali and alkaline earth sulfites and bisulfites, and related compounds to the waste water enhances the catalytic oxidation of cyanide, but also increases the oxygen requirements of the total COD reduction system. The concentration of the cyanide or other offensive substance in the waste water may be too low to treat economically using conventional means, but still too high to meet effluent limitations. other problems with conventional waste water treatment processes for streams having relatively low concentrations of offensive substances include high cost, high severity, and the potential for the introduction of undesirable substances into the treated waste water, such as intentionally added SO.sub.2 type compounds and the soluble metal catalysts mentioned above. Even where water insoluble metal catalysts are used, a relatively small ammonia concentration in the waste water tends to form ammonia-metal complexes and to leach the metal from the catalyst into the treated waste water. Also, many of the previously known processes for waste water oxidation may be intolerant of process upsets, for example, significant pH changes may destroy the catalyst, possibly through leaching, precipitation, or destruction of the physical integrity of the catalyst.
For the above reasons, it is desired to develop an inexpensive, mild, catalytic waste water oxidation process that is effective to remove low concentrations of offensive substances and is sufficiently robust to allow recovery from process upsets without replacement of the catalyst. More specifically, it is desired to develop a rejuvenative, inexpensive process to catalytically oxidize low concentrations of offensive substances which leaves a low concentration of metal compound in the treated waste water.